1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to imaging of lithographic printing-plate constructions on- or off-press using digitally controlled laser output.
2. Description of the Related Art
In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to inking. The ink-abhesive fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
To circumvent the cumbersome photographic development, plate-mounting and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers. For example, U.S. Pat. Nos. 5,351,617 and 5,385,092 (the entire disclosures of which are hereby incorporated by reference) describe an ablative recording system that uses low-power laser discharges to remove, in an imagewise pattern, one or more layers of a lithographic printing blank, thereby creating a ready-to-ink printing member without the need for photographic development. In accordance with those systems, laser output is guided from the diode to the printing surface and focused onto that surface (or, desirably, onto the layer most susceptible to laser ablation, which will generally lie beneath the surface layer).
U.S. Pat. No. 5,339,737, Re. 35,512, 5,783,364, and 5,807,658, the entire disclosures of which are hereby incorporated by reference, describe a variety of lithographic plate configurations for use with such imaging apparatus. In general, the plate constructions may include a first, topmost layer chosen for its affinity for (or repulsion of) ink or an ink-abhesive fluid. Underlying the first layer is an image layer, which ablates in response to imaging (e.g., infrared, or "IR") radiation. A strong, durable substrate underlies the image layer, and is characterized by an affinity for (or repulsion of) ink or an ink-abhesive fluid opposite to that of the first layer. Ablation of the absorbing second layer by an imaging pulse generally weakens the topmost layer as well. By disrupting its anchorage to an underlying layer, the topmost layer is rendered easily removable in a post-imaging cleaning step. This creates an image spot having an affinity for ink or an ink-abhesive fluid differing from that of the unexposed first layer, the pattern of such spots forming a lithographic plate image.
An accepted approach to cleaning involves subjecting the imaged plate to mechanical action, e.g., rubbing or wiping with a cloth, or the rotation of a brush (see U.S. Pat. No. 5,148,746). Mechanical action can occur under dry conditions or be accompanied by a cleaning fluid. In the latter case, the fluid assists in the cleaning process, reducing the amount and intensity of mechanical friction necessary to remove debris and, as a result, lessening the chance of damage to the intact top layer. The cleaning fluid is generally a non-solvent for that layer, once again in order to avoid damage to unimaged areas. In particular, dry plates utilize silicone top layers, which are permeable to various solvents and tend to "swell" under their influence, resulting in weakened anchorage to underlying layers and, consequently, reduced plate durability and performance. Unfortunately, the need to preserve the silicone layer can limit the overall degree of cleaning effectiveness. Without complete removal of imaging byproducts and other pyrolitic debris from imaged portions of the plate, the necessary affinity difference between ink-repellent and ink-accepting layers cannot be achieved.